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Mixing light - mixing paint

Physics Narrative for 11-14 Supporting Physics Teaching

Mixing light

The outcome of mixing paint is not necessarily the same as mixing light. Let's consider one specific example: mixing yellow and blue. Mixing light beams result: yellow + blue  →  white

Mixing paint result: yellow + blue  →  green

Puzzled: Why is there a difference?

In mixing yellow and blue light, you need to remember that yellow can be produced as a secondary colour, containing the primary colours red and green. Adding blue to yellow therefore gives:

Mixing light beams result

yellow (red + green)+blue  →  white

What happens when you mix paint

Mixing paint is different because the process involves removing frequencies (rather than adding frequencies, as with light).

Explanations for the colours when you mix paint

The blue paint absorbs a range of frequencies of the white light falling on it. The remaining frequencies are reflected to make the paint appear blue. In practice the blue paint will reflect frequencies corresponding to the spectral colours blue, some green and some indigo or violet, with the result that the overall colour appears blue.

The yellow paint also absorbs a number of frequencies of the white light falling on it. The frequencies that are reflected make the paint appear yellow. In practice, the yellow paint will reflect frequencies corresponding to the spectral colours yellow, some orange and green, with the result that the overall colour appears yellow.

White light, black paint

It is an interesting fact that mixing all of the colours of the spectrum:

  • With paints (mixing them in a palette) gives black.
  • With light (shining them onto a screen) gives white.

Can you explain why this happens?

With the mix of paints, all of the colours are represented and so all components are removed (or absorbed) from any white light that is incident on the paint. The mix of paints therefore appears black (no light is reflected).

With the mix of light all of the colours add together to produce white.

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